Compressor control arrangement



April 16, 1968 l. BERGER COMPRESSOR CONTROL ARRANGEMENT 2 Sheets-Sheet 1 Filed Aug. 1, 1966 FIG. I

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J 15 7 7 3 NF 2 E El April 16, 1968 BERGER COMPRESSOR CONTROL ARRANGEMENT 2 Sheets-Sheet 2 Filed Aug. 1, 1966 INVENTOR. ISAAC BERGER. W2 ATTORNEY.

United States Patent 3,377,316 COMPRESSOR CONTROL ARRANGEMENT Isaac Berger, Dewitt, N.Y., assignor to Carrier Corporation, Syracuse, N.Y., a corporation of Delaware Filed Aug. 1, 1966, Ser. No. 569,125 10 Claims. (Cl. 62-157) This invention relates to a control arrangement for compressors and, more particularly, to a control arrangement [for compressors adapted for use with refrigeration systems.

Starting loads imposed on the compressor motor due to the relatively high pressure differential that can exist across the compressor may damage and ultimately fail the compresor motor. This situation is particularly evident in the so called pump down type of system where a valve interrupts the refrigerant circuit upon satisfaction of the system demand while the compressor remains energized until pressures at the suction side thereof become low enough to mandate stopping of the compressor. The consequent pressure differential across the compressor at shutdown is substantial and on restart, the compressor motor, which must overcome the pressure differential across the compressor, may be overloaded.

In larger more expensive systems, a multitude of protective devices designed to stop the compressor at various system or compressor malfunctions are often included. However certain malfunctions, as for example loss of compressor oil pressure, are more serious than others, such as abnormal discharge pressures. When a serious malfunction occurs it is desirable to prevent any restart of the compressor until the compressor is serviced. Where a less serious malfunction occurs, however, attempted restarting of the compressor is desired to prevent nuisance service calls.

It is a principal object of the present invention to provide a new and improved compressor control arrangement.

It is a further object of the present invention to provide a control which assures at least partial equalization of pressures across the refrigeration system compressor prior to startup of the compressor.

It is an object of the present invention to provide a control effective to prevent starting of a refrigeration system compressor when pressures at the suction side of the compressor are below a preselected minimum.

It is an object of the present invention to provide a control arangement for refrigerant compressors which deenergizes the compressor at the happening of any one of a number of malfunctions, the control arrangement being adapted at certain of the malfunctions to prohibit restart of the compressor.

This invention relates to a refrigeration system in which comprises, in combination, at least one compressor, a condenser, expansion means, and an evaporator arranged in a closed refrigerant fiow path; valve means closing the refrigerant flow path; a first circuit for energizing the compressor; a second circuit for actuating the valve means to open the refrigerant path; means for completing the second circuit whereby the refrigerant flow path is opened to permit pressures across the compressor to equalize; control means for actuating the second circuit completing means at a predetermined cooling demand; and system pressure sensitive means efliective when system pressures across the compressor are substantially equalized to complete the first circuit whereby the compressor is energized.

Other objects will be apparent from the ensuing description and drawings in which:

FIGURE 1 is a schematic representation of a refrigeration system incorporating the control arrangement of the present invention;

FIGURE 2 is a wiring diagram of the control arrangement of the present invention; and

FIGURE 3 is a wiring diagram of a modified control arrangement of the present invention.

Referring to FIGURE 1 of the drawings, there is shown a refrigeration system incorporating the control arrangement of this invention. The system includes an outdoor heat exchange coil or condenser 2 connected by means of line 3 with the discharge side of a suitable refrigerant compression mechanism, for example reciprocating compressors 4, 5. The gaseous refrigerant from compressors 4, 5 flowing through outdoor coil 2 is condensed by ambient air from outdoor fan 6. Liquid refrigerant from coil 2 flows through line 7 and thermal expansion valve 8 to evaporator 10. It is understood that other suitable expansion devices, i.e., capillary tube may be employed in place of expansion valve 8.

Liquid refrigerant in indoor coil 10 is vaporized through contact with the fluid being conditioned, i.e., water. Vaporous refrigerant from coil 19 returns through line 13 to compressors 4, 5. Control valve 15 is provided in refrigerant line 7. Solenoid 15 opens valve 15 when energized.

Referring to FIGURE 2 of the drawings, the energizing windings of compressor drive motors 16, 17 are connected through contacts 19, 20 respectively across lead L L L Leads L L L are connected to a suitable source of alternating current power (not shown). It is understood that a two phase source of electrical power may be employed if the circuit is suitably modified. Outdoor fan 22 is connected by contacts 25 across leads L L L Thermostatic switch 27, which responds to the cooling load imposed on the system, and control relay 28 are series connected across secondary winding 30 of step-down transformer 29. Primary winding 31 of transformer 29 may be connected across leads L L Transformer 29 may be omitted if desired and the various control circuits connected directly across power leads L and L Solenoid 15' of valve 15 is series connected with solenoid relay switch 33 and thermostatic switch 27.

Compressors 4, 5 preferably include suitable crankcase heaters 35, 36 respectively. Heaters 35, 36 are series connected with switches 37, 38 respectively and timer contacts 39, 80' across transformer secondary winding 30.

Timer motor 40 is series connected with timer switch arm 39, safety switches 45, and control switch 32 across transformer secondary winding 30. In the exemplary ar rangement shown, safety switches comprise switches 46, 46 of compressor oil pressure sensors 44, 44, compressor motor winding temperature responsive switches 47, 47', discharge line temperature switch 48, system high pressure switch 49, and system low pressure switch 50. Safety switches 45 protect the refrigeration system, and particularly the compressors 4, 5 thereof, against various system and compressor malfunctions.

Switches 47, 47' are arranged in intimate heat exchange relation with the windings of compressor motors 16, 17 respectively. Switches 47, 47 open at the happening of a potentially damaging motor winding temperature in the compressor associated therewith. Switch 48, arrange-d in heat exchange relation with discharge line 3, opens at a predetermined high refrigerant temperature.

Pressure switches 49, which sense refrigerant pressure conditions in discharge and suction lines 3, 13 respectively, have internal actuating pressure differentials. Thus, switch 49 which opens at a predetermined high pressure, closes at a lower system pressure. Switch 50, at a predetermined low refrigerant pressure, opens. On a rise in refrigerant pressures, switch 50 closes. Switch 50, when nominally open, closes contact 50' to equalize system pressures before startup of the compressors 4, 5 as will be apparent hereinafter. Solenoid drop relay and are series connected with low )I'GSSUIC switch contact 55'. Contact 56 is normally closed.

Compressor oil pressure sensors 44, 44 are thermal )owered devices requiring, on opening of bimetal switch #6, 46 thereof, manual resetting. Heater 57 of sensor 44 is series connected with pressure switch 58, switch arm 59 3f timer 4t), switch 71, protective switches 45, and control switch 32 across transformer secondary winding 36.

eater 57' of sensor 44, pressure switch 5%, switch arm 81 of second timer 40', switch 76, capacity control switch 60, protective switches 45, and control switch 32 are series connected across transformer winding 36. Pressure switches 58, 58' are differential pressure type devices which open when oil pressures of compressors 4, 5 respectively reach a safe operating level.

At startup of compressors 4, 5 heaters 57, 57' of sensors 44, 44' respectively are energized. Upon attainment of proper oil pressures, switches 58, 58 interrupt the energizing circuit to heaters 57, 57. If oil pressure in one of compressors 4, 5 is insufficient to open the switch 58 or 58 associated therewith, the affected heater 57 or 57, following a short delay, opens its bimetal switch 46 or 46 to deenergize both compressors 4, 5 and prevent restarting thereof until the affected sensor 44, 46 is manually reset.

Compressor contactor 65 is series connected with control switch 32, protective devices 45, switch 71 and timer switch 59 across transformer winding 30. Contactor 65, when energized, closes contacts 19, 69 and opens contact 56. Compressor contactor 66 is series connected with control switch 32, protective devices 45, capacity control switch 60, switch 76 and second timer switch 81 across transformer winding 3%. Contactor 66 upon energization closes contacts 20 to energize compressor motor 17. Capacity control switch 60 closes on increased system load and may be responsive to pressure conditions in line 13 of the refrigeration system.

Solenoid drop relay 55, control switch 68 and compressor contact 69 are connected in parallel with compressor contactor 65. Timer relay 7% and timer relay switch 71 thereof are series connected with control switch 32. and protective switches 45. A timer relay switch 72 is arranged in parallel with control switch 32. A second timer relay 75 is series connected with second timer relay switch 76, capacity control switch 60, protective devices 4-5, and control switch 32.

On a demand for cooling, thermostatic switch 27 completes a circuit to control relay 28 which closes control switches 32, 68. Assuming previous operation of the system to have been terminated by low pressure switch 50, switch 50 is open. However, contact 50' of switch 50 is closed. On closure of control switch 32, solenoid drop relay 55 is energized through normally closed compressor contactor 56. Relay 55 closes switch 33 to complete an energizing circuit to solenoid 15' which opens valve 15. Opening of valve 15 equalizes system pressures across the compressors by permitting relatively high pressure refrigerant in coil 2 and lines 3 and 7 to feed through expansion means 8 into the relatively low pressure coil 10 and line 13.

When the pressures in line 13 reach the pressure reqired to close low pressure switch 50, switch 50 completes, through contact 50" thereof, control switch 32, safety switches 45, which are normally closed, and timer switch arm 39, an energizing circuit to timer motor 46. Additionally, there is completed energizing circuits to contactor 79 which closes contacts 25 to energize fan 22 and through timer switch arm 59, to timer relay 70 to close switches 71, 72 and open switch 37. Switch 37 interrupts the energizing circuit to crankcase heater 35.

Low pressure switch 50, on closing, opens contact 56' to interrupt the energizing circuit to solenoid drop relay 55 which in turn deenergizes solenoid permitting valve 15 to close. Since system pressures across compressor 4 are substantially equalized at the time of closure of switch 50, reclosure of valve 15 is permitted.

:ompressor contact 56 Following a short interval, timer moves switch arm 59 thereof to close contact 59 and complete an energizing circuit to compressor contactor 65. Contactor 65 closes contacts 19, 69 and opens contact 56. Contacts 19 complete the energizing circuit to compressor motor 16.

Additionally, timer contact 59' completes circuits through control switch 68 and compressor contact 69 to solenoid drop relay and through pressure switch 58 to heater 57 of oil pressure sensor &4. Relay 55 energizes solenoid 15 which opens valve 15 as described heretofore.

leater 57 of pressure sensor 44 commences heating bimetal switch 46 thereof. Assuming adequate oil pressure to have been established in compressor 4 following energization of motor 16 thereof, pressure switch 58 opens to interrupt the circuit to heater 57. Following a second timed interval, timer switch arm 39 closes contact 39' to interrupt the energizing circuit to timer motor 40.

If the load imposed on the system is greater than that capable of being met by compressor 4- only, pressures in line 13 of the system rise and at a predetermined system pressure, capacity control switch closes to complete an energizing circuit through second timer switch arm 30 to timer motor 46. At the same time, closure of capacity control switch 66 completes an energizing circuit through switch arm 81 to second timer relay 75. Timer relay 75 closes switch 76 and opens switch 38, the latter interrupting the energizing circuit to crankcase heater 36 of compressor 5.

Following a brief interval, timer motor 40' moves switch arm 81 to close contact 81" and complete an energizing circuit to compressor contactor 66, and through pressure switch 58' to heater 57' of sensor 44. Contactor 66 closes contacts 26 to energize motor 17 of compressor 5. On the establishment of adequate compressor oil pressure, switch 58' opens to interrupt the energizing circuit to heater 57' of sensor 44'. Following a second interval of time, timer switch arm 8% closes contact 80' thereof to deenergize second timer motor 40'. The system accordingly operates at full capacity.

If at startup of either compressor 4 or 5, pressure switch 58 or 58 thereof fails to open within the interval required for oil pressure sensor heater 57 or 57 to open bimetal switch 46 or 46', the affected switch 46 or 46 opens to deenergize compressor contactors 65, 66, solenoid drop relay 55 and timer relays 70, 75. Deenergization of contactors 65, 66 interrupts the energizing circuits to motors 16, 17 of compressors 4, 5 respectively. Deenergization of relay 55 opens switch 33 to interrupt the circuit to solenoid 15 closing valve 15 to forestall thermodynamic migration of refrigerant from condenser 2 to evaporator 10 and compressors 4, 5.

As noted heretofore, oil pressure sensors 44, 44' require manual resetting. Accordingly, further operation of the system is prevented until the affected sensor 44- or 44' is reset.

The deenergization of timer relay 70 opens switches 71, 72 while closing switch 37 thereof. Switch 37 completes an energizing circuit to crankcase heater 35 and, through timer switch arm 39, to timer motor 40. Following a relatively long interval, timer motor 46 moves switch arm 39 to close contact 39" and open contact 39 whereupon motor 40 is deenergized. Simultaneously motor 40 moves switch arm 59 to close contact 59 to ready the compressor 4- for restart upon resetting of the affected oil pressure protector. In a similar manner, deenergization of timer relay 75 opens switch 76 while closing switch 38 thereof. Switch 38 energizes crankcase heater 36 of compressor 5 and, through switch arm 86, energizes timer motor 46' to reset timer switch arms 80, 81.

Should motor winding temperature responsive switch 47, 47', discharge line thermostatic switch 48, system high pressure switch 49, or system low pressure switch 50 open, compressor contactor coils 65, 66, solenoid drop relay 55, and timer relays 70, 75 are deenergized. Compressors 4, 5

are deenergized and system valve closed while crankcase heaters 35, 36 are energized as explained heretofore. Timer switch arms 39, 59 and 80, 81 are reset.

On reclosure of the affected protective switch, and assuming control switch 32 is closed, a circuit is completed through switch 32, safety switches 45, and switch arm 39 to timer motor 40. Additionally, timer relay 70 is energized through timer switch arm 59. Compressor 4 is restarted in the manner explained heretofore.

Should capacity control switch 60 'be closed, energizing circuits are completed through timer switch arms 80 and 81 to energize second timer motor 40', timer relay 75, and second compressor 5. Preferably, the timer motor 40' includes a time delay to prevent simultaneous startup of compressor 5 with compressor 4.

On satisfaction of the cooling demand imposed on the system, thermostatic switch 27 opens interrupting the energizing circuits to control relay 28 and solenoid 15. Deenergization of control relay 28 opens control switches 32, 68. Deenergization of solenoid 15' permits valve 15 to close.

The continued operation of compressor 4, and compressor 5 when actuated, reduces or pulls down pressures in system suction line 13. When pressures in line 13 fall below the pressure setting of low pressure switch 50, switch 50 opens to interrupt the energizing circuits to compressor contactors 65, 66, timer relays 70, 75 and fan contactor 79. Deenergization of contactors 65, 66 renders compressors 4, 5 inoperable. Similarly, deenergization of contactor 79 renders fan 22 inoperative. Deenergization of timer relays 70, 75 energizes crankcase heaters 35, 36 of compressors 4, 5 respectively and resets timer switch arms 39, 59 and 80, 81 as explained heretofore.

In the modified control arrangement illustrated in FIG- URE 3 of the drawings, where like numerals refer to like parts, each timer motor 40, 48' has a third switch arm 85, 86 respectively series connected with motor winding temperature responsive switches 47, 47' between first compressor contactor 65 and first timer switch arm 59. Oil pressure switches 88, 89 are connected in parallel with switch arms 85, 86 respectively. Pressure switches 88, 89 sense oil pressures of compressors 4, 5 respectively, the switches 88, 89 closing at a selected oil pressure in the compressor associated therewith. Normally closed switch 96 of second timer relay 75 parallels second timer switch arm 86.

Switch 91 of compressor contactor 65 is connected in series with second compressor contactor 66 and switch arm 81 of timer 40. To enable compressors 4, 5 to be restarted following opening of one of switches 47, 47', 88 or 89, normally closed reset switch 93 is in series with switches 48, 49, 58.

On a demand for cooling, thermostatic switch 27 closes to complete an energizing circuit to control relay 28 which in turn closes control switches 32, 68. Solenoid drop relay 55 is energized via control switch 32, low pressure switch contact 50' and contact 56. Solenoid drop relay 55, acting through solenoid 15', opens valve 15 to effect system pressure equalization prior to compressor startup. At a predetermined system pressure, low pressure switch 50 closes.

Closure of switch 50 completes a circuit through control switch 32, low pressure switch 50, high pressure switch 49, thermostatic switch 48, reset switch 93, and timer switch arm 39 to timer motor 40. Timer 40 closes switch arm 85.

Following a short interval, timer motor 40 moves switch arm 59 to close contact 59' to complete circuits to timer relay 7t) and, through switch arm 85, temperature protectors 47, 47 and normally closed switch 90, to compressor contactor 65. Relay 70 closes switches 71, 72 and opens switch 37 to interrupt the energizing circuit to crankcase heater 35. Contactor 65 closes contacts 19 to energize motor 16 of compressor 4. Additionally, contactor 65 closes contact 69 to complete, through switch arm 59, contact 69 and control switch 68, an energizing circuit to solenoid drop relay 55. Contactor 65 opens contact 56 and closes contact 91.

Timer switch arm 85 is held closed for a relatively short interval to permit oil pressure build up in compressor 4. At the preselected minimum oil pressure, switch 88 closes. Should pressure switch 88 fail to close before timer switch arm 85 opens, at the opening of switch arm 85, the circuit to contactor 65 is interrupted and compressor motor 16 is deenergized.

Timer switch arm 39 thereafter opens contact 39" thereof and closes contact 39" to deenergize motor 40.

Closure of capacity control switch 60 in response to increased load energizes, through switch arm 80, second timer motor 40'. Timer motor 40' closes switch arm 86 for a short interval bypassing second compressor pressure switch 89. Thereafter switch arm 81 is moved to close contact 81 to complete energizing circuits to second timer relay 75 which closes switch 76 and opens switches 38, 98 and through first compressor contact 91, to second compressor contactor 66 which closes contacts 20 to energize compressor motor 17. Opening of switch 38 dedenergizes crankcase heater 36.

Following a short interval, switch arm 86 of timer 40' is opened. Assuming adequate oil pressure in compressor 5, switch 89, now closed, sustains energization of contactor 65. Following a further time interval, timer switch arm opens contact 80" to deenergize timer motor 40.

Should pressure switches 88, 89 fail to close at startup or open subsequently, or should winding temperature responsive switches 47, 47' open, the circuit to first compressor contactor 65 is interrupted. Deenergization of contactor 65 closes contact 56 and opens contacts 19, 69 and 91 thereof. Contact 91 interrupts the energizing circuit to second compressor contactor 66 which in turn opens contacts 20. Opening of contacts 19, 20 deenergize compressor drive motors 16, 17 while opening of contact 69 deenergizes solenoid drop relay 55 to close valve 15.

Timer relay 70 remains energized through the circuit comprising switch 72, low pressure switch 50, high pressure switch 49, thermostat switch 48, reset switch 93, and and switch 71. Relay 70, while energized, prevents any restart of compressors 4, 5 by preventing, through switch 37 thereof, completion of an energizing circuit to first timer motor 40. Timer switch arm 39, reset following startup of compressor 4, holds contact 39' thereof closed. Similarly, if capacity control switch 60 had been closed, second timer relay 75 is held energized through switch 76 thereof. Relay 75 holds switch 38 thereof open preventing energization of second timer motor 40'. By this arrangement, any restart of compressors 4, 5 following opening of one of switches 47, 47', 88 or 89 is prevented.

To permit restart following opening of one of switches 47, 47' 88 or 89, reset switch 93 is manually opened for a short interval to interrupt the energizing circuit to timer relay 76 thereby permitting closure of switch 37 and completion of an energizing circuit through timer switch 39 to first timer 40. With the energization of first timer 40, the compressor 4 is restarted in the manner explained heretofore.

Opening of low pressure switch 50, high pressure switch 49, or discharge thermostat 48 interrupts the energizing circuits to contactors 65, 66, solenoid drop relay 55 and timer relays 70, 72 to deenergize compressors 4, 5 in the manner explained heretofore. On reclosure of the affected switch 50, 49 or 48, a restart of the compressor through the timing mechanism is automatically effected.

It is understood that discharge line thermostatic switch 48 and/or system high pressure switch 49 may be series connected with oil pressure switches 89, 88 and motor winding temperature switches 47, 47' in the line to first compressor contactor 65. Alternately, switches 47, 47 may be positioned in series with protective switches 48, 49. In this latter instance, opening of either switch 47 or 47' in response to an abnormal motor winding temperature and subsequent closure of the affected switch permits automatic restarting of the compressors 4, 5 following the time delay enforced by timers 40, 40 under the control of bimetal switch 27 in accordance with cooling demand.

While the exemplary wiring diagrams illustrate two compressors 4, 5, it is appreciated that the control arrangements are readily usable for one or more compressors. Where a single compressor is controlled, second timer motor 40, contactor 66, timer relay '75, and the switches associated therewith, crankcase heater 36, and winding overtemperature protector 47' are omited. And, in the FIGURE 2 circuit, oil pressure sensor 44, including bimetal switch 46, heater 57' and pressure switch 58 thereof are omitted.

In the FIGURE 3 control circuit the second compressor oil pressure switch 89 and contact 91 of first compressor contactor 65 are omitted for single compressor control.

Applicant has provided a controlling arrangement for refrigerant system compressors which, by use of the system pressure switch, effectively equalizes the system pressure differential across the compressor prior to startup thereof. Additionally, applicant provides a compressor control effective on the happening of potentially extremely serious malfunction to prevent any restarting of the compressor until serviced yet which at less serious malfunctions permits automatic restart of the compressor.

While I have described a prefered embodiment of my invention, it will be understood that my invention is not limited thereto, but may be otherwise embodied within the scope of the following claims.

I claim:

1. In a refrigeration system having at least one compressor, a condenser, expansion means, and an evaporator arranged in a closed refrigerant flow path, and valve means closing refrigerant flow path, the combination of: a first circuit for energizing said compressor; a second circuit for actuating said valve means to open said refrigerant flow path; means for completing said second circuit whereby said refrigerant fiow path is opened to permit system pressures across said compressor to equalize; control means for actuating said second circuit completing means at a predetermined cooling demand; and system pressure sensitive means effective when system pressures across said compressor are substantially equalized to complete said first circuit whereby said compressor is energized.

2. A refrigeration system according to claim 1 including means for interrupting said second circuit whereby said valve means closes said refrigerant flow path, said control means actuating said second circuit interrupting means upon satisfaction of said cooling demand, said system pressure sensitive means being adapted upon a predetermined increase in system pressure differential across said compressor following closure of said refrigerant flow path to interrupt said first circuit and deenergize said compressor.

3. A refrigeration system according to claim 2 including means for bypassing said second circuit completing means to render said second circuit completing means ineffective, said bypassing means including an actuating relay and a third circuit effective when completed to energize said relay; said system pressure sensitive means being adapted to complete said third circuit when pressures across said compressor are substantially equalized following actuation of said valve means.

4. A refrigeration system according to claim 3 including timing means effective following a timed interval to interrupt said first circuit and deenergize said compressor, said timing means preventing reclosure of said first circuit and restart of said compressor; an energizing circuit for said timing means; and compressor oil pressure sensitive means adapted when compressor oil pressures are below a predetermined minimum to complete said timing means circuit so that when compressor oil pressures are below said predetermined minimum pressure for said timed interval, said timing means interrupts said first circuit to deenergize said compressor.

5. A refrigeration system according to claim 3 including means for sensing compressor oil pressure conditions adapted when compressor oil pressures are below a preset minimum to interrupt said first circuit and deenergize said compressor.

6. A refrigeration system according to claim 5 including timing means effective at startup of said compressor to bypass said oil pressure sensing means for a. timed interval to permit initial completion of said first circuit and startup of said compressor; and means for resetting said timing means to permit restarting of said compressor, said resetting means being actuable in response to interruption of said third circuit whereby, on deenergization of said compressor by said oil pressure sensing means, actuation of said resetting means and restart of said compressor is precluded so long as said third circuit is completed.

7. A refrigeration system according to claim 6 including switch means for interrupting said third circuit to actuate said resetting means to permit restarting of said com-pressor, said switch means being arranged for manual operation.

8. A refrigeration system according to claim 7 including means effective at a preset malfunction to interrupt both said first and third circuits, interruption of said first circuit deenergizing said compressor; interruption of said third circuit actuating said resetting means to permit restarting of said compressor.

9. A refrigeration system according to claim 6 including a second compressor; a fourth circuit effective when completed to energize said second compressor; said control means adapted at a second predetermined cooling demand to complete said fourth circuit to energize said second compressor; second compressor oil pressure sensing means adapted when oil pressures of said second compressor are below a preset minimum to interrupt both said first and fourth circuits whereby both said first and second compressors are deenergized.

10. A refrigeration system according to claim 9 including a circuit for bypassing said second compressor oil pressure responsive means to permit operation of said first compressor only; means adapted at energization of said second compressor to interrupt said bypass circuit; and second timing means effective at startup of said sec and compressor to bypass said second compressor oil pressure sensing means for a timed interval to prevent interruption of said first circuit and permit initial completion of said fourth circuit at startup of said second compressor.

References Cited UNITED STATES PATENTS MEYER PERLIN, Primary Examiner. 

1. IN A REFRIGERATION SYSTEM HAVING AT LEAST ONE COMPRESSOR, A CONDENSER, EXPANSION MEANS, AND AN EVAPORATOR ARRANGED IN A CLOSED REFRIGERANT FLOW PATH, AND VALVE MEANS CLOSING REFRIGERANT FLOW PATH, THE COMBINATION OF: A FIRST CIRCUIT FOR ENERGIZING SAID COMPRESSOR; A SECOND CIRCUIT FOR ACTUATING SAID VALVE MEANS TO OPEN SAID REFRIGERANT FLOW PATH; MEANS FOR COMPLETING SAID SECOND CIRCUIT WHEREBY SAID REFRIGERANT FLOW PATH IS OPENED TO 